Bruton's Tyrosine Kinase (BTK) is a member of the Tec family kinases with a well-characterized role in B-cell receptor (BCR)-signaling and B-cell activation (Schwartzberg, P. L., Finkelstein, L. D and Readinger, J. A., Nat. Rev. Immunol., 2005, 5, 284-295). BTK is a key signaling enzyme expressed in all hematopoietic cells types, except T lymphocytes and natural killer cells. BTK is activated by the upstream Src-family kinases Blk, Lyn and Fyn and it lead to downstream activation of essential cell survival pathways such as NF-KB and MAPK (Afar, D. E., Park, H., Howell, B. W., Rawlings, D. J., Cooper, J and Witte, O. N., Mol. Cell Biol., 1996, 16(7), 3465-3471). In turn, BTK phosphorylates and activates phospholipase-C (PLC), leading to Ca2+ mobilization and activation of NF-KB and MAP kinase pathways (Bajpai, U. D., Zhang, K., Teutsch, M., Sen, R and Wortis, H. H., J. Exp. Med., 2000, 191, 1735-1744).
BTK is intimately involved in multiple signal-transduction pathways regulating survival, activation, proliferation and differentiation of B-lineage lymphoid cells. BTK is an upstream activator of multiple antiapoptotic signaling molecules and networks, including the signal transducer and activator of transcription 5 (STATS) protein, phosphatidylinositol (PI) 3-kinase/AKT/mammalian target of rapamycin (mTOR) pathway, and nuclear factor kappa B (NF-κB) (Mahajan, S., Vassilev, A., Sun, N., Ozer. Z., Mao, C and Uckun, F. M., J. Biol. Chem., 2001, 276, 31216-31228; Ortolano, S., Hwang, I. Y., Han, S. B and Kehrl, J. H., Eur. J. Immunol., 2006, 36, 1285-1295). This downstream signal transduction protein is a critical effector molecule that governs normal B-cell development, differentiation and functioning and has also been implicated in initiation, survival and progression of mature B-cell lymphoproliferative disorders (Kuppers, R., Nat. Rev. Cancer, 2005, 5(4), 251-262).
In B-cells, BTK is important for B-cell antigen receptor-, CD40- and Toll-like receptor 4-mediated activation and proliferation. Furthermore, BTK plays a role in B-cell antigen processing and presentation (Satterthwaite, A. B., Witte, O. N., Immunol. Rev., 2000, 175, 120-127). It is noteworthy that BTK is also essential in Fc receptor-mediated inflammatory cytokine production [tumor necrosis factor , interleukin (IL)-1 and IL-6] in monocytes/macrophages and therefore can contribute to immune complex-induced disease. BTK is abundantly expressed in malignant cells from patients with B-cell precursor (BCP)-acute lymphoblastic leukemia (ALL; the most common form of cancer in children and adolescents), chronic lymphocytic leukemia (CLL), and non-Hodgkin's lymphoma (NHL) (Khan, W. N., Immunol. Res., 2001, 23, 147-156). Consequently, BTK has emerged as a new molecular target for treatment of B-lineage leukemia's and lymphomas.
BTK mutations in human cause an inherited disease X-linked agammaglobulinemia, characterized by a lack of peripheral B-cells and low levels of serum Ig. Thus, BTK is a uniquely attractive kinase target for selective B-cell inhibition and small molecule based BTK inhibitors offers therapeutic benefit in the treatment of lymphoma and autoimmune diseases (Valiaho, J., Smith, C. I., Vihinen, M., Hum. Mutat., 2006, 27(12), 1209-1217).
BTK was recently identified in a siRNA screen as an essential kinase for survival in a subset of diffuse large-cell lymphomas driven by activated BCR, where an irreversible BTK inhibitor, PCI-32765 (Ibrutinib), was shown to promote apoptosis. A second study of Ibrutinib recently demonstrated in vivo clinical responses in dogs with aggressive B-cell lymphomas (Honigberg, L. A., Smith, A. M., Sirisawad, M., Verner, E., Loury, D., Chang, B., Li, S., Pan, Z., Thamm, D. H., Miller, R. A., Buggy, J. J., Proc. Natl. Acad. Sci., USA., 2010, 107(29), 13075-13080).
Thus BTKs are important in the regulation of many cellular processes including cell cycle regulation, proliferation, survival, apoptosis and motility and are significant components of the molecular mechanisms of diseases such as cancer, diabetes and immune inflammation (Buggy, J. J., Elias, L., Int. Rev. Immunol., 2012, 31(2), 119-132).